Hydrophilic polyurethane resin coatings function by absorbing moisture on the side exposed to high humidity and discharging moisture from the side exposed to low humidity; that is they are moisture vapor permeable. This property is known as breathability. Breathability is a very desirable comfort factor for clothing.
Breathability is achieved in urethane polymers by incorporating in them polyols which contain major portions of alkylene oxide units, usually ethylene oxide units. With urethane coatings containing about 55% of oxyethylene units by weight, the minimum desired vapor transmission rate can be achieved. Other chemical factors also appear to affect the moisture vapor transmission rate. For instance, increased cross linking in the polymer appears to lower the moisture vapor transmission rate.
The breathability of the urethane polymer can be transferred to porous hydrophobic substrates, such as expanded porous polytetrafluoroethylene as described in U.S. Pat. Nos. 3,953,566, 4,194,041, 4,187,390, 4,110,392, 4,482,516, and 4,025,679 by either applying a hydrophilic coating to the surface of the substrate or by casting a film of the hydrophilic resin on a release paper and then adhering the film to the surface of the substrate. It is to the formation of such coatings and films that this invention is directed. Not only do the coatings and films need to be highly breathable, as measured by the moisture vapor transmission rates, but their precursors, the prepolymers from which they are formed need to have selected properties as well. Among the several highly desirable properties in the prepolymers are:
1. They should have very low melting points, preferably below 0.degree. C. Crystallization or precipitation in the coating mixture leads to flaws and pinholes during coating operations. Such coatings must be scrapped. Operation at increased temperatures to ensure all of the material is molten leads to increased levels of toxic isocyanate monomers in the air.
2. The liquid prepolymers should have low viscosities, preferably 15,000 cps or lower at 25.degree. C. This reduces or eliminates need for solvents, which are usually used to thin the coating mixture to usable viscosities. The environmental factors which are affected by organic solvents in the atmosphere are well known and pressures are constantly being applied to reduce their emissions.
3. They should produce final coatings or films with good breathability, i.e., with moisture vapor transmission rates of at least 5000 g/m.sup.2 /day on 5 to 10 mil thick films. This can be achieved only with high levels of oxyethylene groups in the final polymer.
4. They should contain less volatile, and therefore less hazardous, diisocyanates and/or very low levels of free isocyanate in the prepolymer. The most common diisocyanate used in coatings is toluene diisocyanate (TDI) which is 200 times more volatile than diphenyl methane diisocyanate (MDI), so the drive to use MDI and other less volatile diisocyanates is very strong. Similar safety benefits can be achieved, however, by reducing the more volatile diisocyanates to very low levels in the prepolymers.
Prepolymers made with MDI tend to be solids, as the MDI molecule is symmetrical, whereas TDI prepolymers tend to be liquids or low melting solids due both to the asymmetry in the TDI molecule and to the fact that commercial TDI is a mixture of its 2,4- and 2,6- isomers.
Keeping the functionality, that is, the number of active groups per molecule of the reaction components down to about 2 (and not above 2.2) is another means to produce prepolymers with low viscosities.
Other workers (European Pat. Appln. No. 0 218 88l) used trifunctional polyols with diisocyanates and included lower molecular weight diols to make hydrophilic, breathable coatings. As is demonstrated in our comparative examples, use of this method with TDI resulted in low water vapor permeability and with MDI the water vapor permeability was low and the viscosity was outside a useable range at 58,500 cps.
The present invention aids in overcoming the above mentioned restrictions and produces low melting, low viscosity prepolymers containing less volatile hazardous isocyanate and which cure to highly hydrophilic coatings and films.